The scientists analyzed the acoustical characteristics of the various screams. And they found these alarming sounds are not only loud, they’re particularly rough. That is, they rapidly change in frequency. Normal speech is relatively smooth, with frequencies ranging only between four and five Hertz, or cycles per second. But a shriek can swiftly fluctuate between 30 and 150 Hertz—a range that can make your hair stand on end.

When the researchers asked people to rate how frightening screams were, they found that more roughness meant more terrifying. And increasing roughness of the sound corresponded to greater activation of the fear center of the brain.

Only one other noise scored as high as screams in terms of roughness [car alarm sound]. Which, ironically, is enough to make you [scream sound].

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